How to add/modify "one click checkout"-page

[kansas3d]

How can I add something in the "one click checkout"-page?

 

I would like to add the "create your account"-box in it.

(The "create your account"-box is the short login box you get when you click on "Log in" at the top of the website)

 

You can see in the picture some closer details.

 

 

Thank you